I remember reading somewhere than ELF [Extremely Low Frequency] radio transmission is inefficient because it requires to much power.
If that is the case, wouldn't MW [Medium Wave] radio transmission require even more power?
MW and ELF are forms of electromagnetic radiation in the RF spectrum.
An photon [or electromagnetic wave] of a higher-frequency has more energy than a photon of a lower-frequency.
Let's say there are there are two radio transmitters, one emits 2 GHz waves while the other emits 2 kHz waves. If the two radio transmitters use the same modulation scheme [AM/FM, etc.] and emit the same amount of photons-per-second-per-square-meter, the 2 GHz transmitter will be using more watts than the 2 kHz transmitter -- because a 2 GHz photon requires more power to generate than a 2 kHZ photon. Right?
So how would transmitting a lower-frequency radio wave require more power than transmitting a higher-frequency radio wave?
I know this is the BASIC channel how ever, All I can say is WOW!
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Of course, that's not an inefficiency of spectrum.
And the inefficiency of the long antennas means nothing since such low frequencies are used for specific purposes where such low frequencies are the only choices. Given that, the only choice is to use such low frequencies, or not communicate at all.
You are making the classic mistake of confusing quality with quantity, energy with power. Which has more pressure, a 55,000 psi water jet or Niagara Falls?
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There is the modulation method - AM or FM. FM is more efficient - at least narrowband FM. That's why we use FM for mobile transmitters. You could use supressed-carrier AM of course but that is a sod to demodulated.
The higher the frequency the shorter the distance it can travel for a given power. Therefore VLF can travel round the world and back again! trouble is you may need for ELF an aerial the size of a mountain range! For a 10 Gig frenquency you would need to pump out a heluva lot of power for it to go any distance. Inverse square law.
Would help of you went to study engineering at Uni - then most of your questions would be answered.
I suspect what you actually read about was the US military scheme to communicate with submerged submarines using high-power VLF that could penetrate seawater. At one point there were plans to use huge underground ore veins in Michigan's Upper Penninsula as the transmitter antenna. That might have been a tad inefficient!
There's also the issue of low carrier frequencies not supporting high symbol rates. I seem to recall that the submerged subs would get only code, at rates so slow even the rankest beginning amateur operator would have had no trouble keeping up...
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Shush Bob, you're deliving into 100% reliable long distance communications techniques first discovered and exploited in the 1950s and which have remained classified ever since. As you point out, the information transmission rate is incredibly low, but extremely reliable and nearly impossible to block or jam, and it reliably reaches every location on earth, surface, underwater, or underground, at extremely low baud rates.
Now what would anyone imagine what purpose such a limited system might be used for? This is precisely why VLF systems of this type remain highly classified. Yes, correct. They have only one practical application. Let's simply call it the Fed Ex principle, which applies to things that must be reliably delivered on schedule.
I'm not sure about output power, but I do know that the lower the frequency, the longer the wavelength, and hence the longer the transmitting antenna. ELF requires a humongous antenna.
It's cheap but not immune to noise and suseptable to multipath big- time.
Do tell how...It's not in any text book so maybe we can learn with your advanced knowledge.
You have never heard of the inverse square law obviously. High frequencies are line of site only and can go long distances because you pump out more power. You need to compare apples with apples.
The only thing we agree on - you must be a physicist - no idea about engineering.
As for your moon thing - it's line of site again!! Try communicating from London to New York at 10GHz.
ELF can be inefficient because the wavelength is so large that it's hard to build an efficient antenna for it. On the transmitting side, this means that only a small amount of the current sloshing around in the antenna gets coupled out into free-space radiation; as a result you need higher currents in the antenna to get the same radiated output, and that means more losses to things like resistive heating.
If you can build an antenna that's appropriately sized for the ELF wavelength (hundreds or thousands of kilometers) then you can avoid this. The US used to have a couple of giant ELF antennas in the Midwest; Wikipedia says they were disassembled earlier this decade. I don't know how submarines are signaled these days.
Anyway, MW has a shorter wavelength than ELF, so it's easier to build a good antenna for that band.
The important thing is usually not how many photons can be emitted, but how much energy can be picked up by the receiver compared to the amount of noise it's also picking up. The amount of energy per photon really isn't significant, at least not for radio. The energy of a single photon is insignificant compared to the power of the Force ... errr ... I mean, it's tiny compared to the amount of energy you need to be heard over the background noise.
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I never said supressed carrier wasn't immune to noise.
As for multipath, all modulation methods are susceptable to it. FM has a slight advantage there with discriminator capture.
See any current amateur radio transceiver. There have been IC's to do it for decades.
The inverse square law applies to isotropic radiators. No real world RF antenna is an isotropic radiator.
Define "high frequencies".
Things don't become line of sight until about 50 Mhz. Most long distance terrestrial communications is done between about 5 Mhz and 30 Mhz, which is HF.
The typical amateur radio transceiver puts out 100 W max in the HF bands.
My log books, and the logs of 100s of thousands of amateur operators are full of contacts around the globe with far less power than 100 W in the 1.6 Mhz to 29 Mhz range.
No, I'm a BSEE and an amateur radio operator for 40 years.
Have you ever seen a HF transmitter much less operated one?
That's exactly the point. You can't communicate anywhere that isn't line of sight much over about 100 Mhz no matter how much power you run unless you use some reflective technique such as tropo scatter.
Sure; for constant photon rates, one transmitter is outputting, say, 1 million watts, and the other is doing 1 watt. But broadcasters don't pay for photons, they pay for watts.
Any transmitter can run at any power level, and photons don't matter. The problem with ELF is that an efficient antenna is enormous, and elf waves don't bounce off the ionosphere like mw waves do, so most of the energy cruises right out into space.
Huge elf rigs were/are used for loran-C, WWVB, and communicating with atomic subs. They use inefficient ground-wave propagation, so need huge power levels. The loran-C station north of San Francisco is about
Not really. They use frequencies that will have good penetration, and which don't suffer much from radio conditions.
Because the frequencies are so low, that ground-wave will be considerable, while higher frequencies need to bounce off the ionosphere and such to get the same sort of distance. But that is unreliable, and of course often is dependent on the time of the day.
Those frequencies are terribly reliable, interference aside.
Once that choice is made, then they have to live with the inefficiencey. They have decided there is no other choice for their needs, and then compensate with the high power to overcome the inefficiency of the antennas.
As an example, there's a broadcast station in Ottawa on 580KHz that comes in fine during the day. But at night, they are required to cut back on their power, so the station goes away, not enough power for ground wave to Montreal, while bouncing off the ionosphere results in a bounce too far away. They have to cut back their power at night so the better propagation at night does not leave them with a booming signal bouncing off the ionosphere to interfere with all the other stations on that frequency. I'm sure there are plenty of locations much further away that can receive the station at night, that can't receive it at night (after all, I can hear plenty of AM broadcast stations at night from much further away that I could never hear during the daytime). I did receive the Ottawa station at night during one period a decade ago, when an emergency situation allowed them to run full power at night; ground wave reception was fine.
So they run WWVB and such at low frequencies so the reliable ground wave is used for really long distances (I can receive it fine here in Montreal, I've had an "atomic clock" for almost five years and rarely does it not sync up at night). One can argue that their high power is not just because of the inefficient antennas at 60KHz, but so it is receivable so far away, just like that Ottawa station where the ground wave signal disappears when they lower their power.
Amateurs are the worst kind! There is not way to demodulate double side-band supressed carrier (esp at low SNRs). The only way (for analogue that is) is to recover the carrier and this cannot be done since the carrier aint there in the first place!
Try locking a PLL into supressed carrier. So you are talking complete rubbish.
Here is the basic euqation
m.cos(wmt)cos(wct) where wm and wc are the modulating and carrier frequencies and m is the amplitude. Actually what you are probably thinking of is where a BFO is multiplied into this. However this is not stable and not phase-locked to the carrier either.It needs constant adjustment though its a lot better than it was because of DDS and stable crystals that we didn't have a long time back. If you try and limit the signal to recover the carrier then it will work at high SNRs but not at low SNRs.
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